1. Technical Field
This invention relates generally to x-ray inspection of semiconductor devices, and more particularly, to prevention of changes to semiconductor device properties while proper x-ray inspection is achieved.
2. Background Art
In order to maximize the quality of printed circuit board manufacturing process, semiconductor devices, in particular surface mounted devices with xe2x80x9chiddenxe2x80x9d solder joints to a printed circuit board, typically undergo an x-ray in on. In f thereof, and with reference to FIG. 1 (rotated 90 degrees clockwise from a conventional orientation of elements therein), a semiconductor device 20 is placed on an inspection tray 22 of for example polymer material. Such a typical semiconductor device 20 includes silicon body 24 having a protective coating 25 of molding compound (In FIG. 1 shown lying on the tray 22), the silicon body 24 having active region 24A and inactive region 24B secured to a substrate 26 by a silver-organic material adhesive 28 (wire bonds connecting silicon body 24 and substrate 26 not shown). The substrate 26 includes organic portions 30, 32 (dielectric layers) and patterned copper layers (one shown at 34), which copper layer 34 communicates with the active region 24A of the silicon body 24 (active region 24A approximately one micron (1xcexcm) in thickness and oriented most adjacent the tray 22) and lead/tin solder balls 36 which connect to a layer of copper traces 38 on an organic material (for example polyimide, epoxy, polyethylene, or glass fiber) board 40, i.e., a printed circuit board or printed wiring board. It will be understood that the particular configuration of the semiconductor device 20 shown is for purposes of illusion, and that such device 20 may be configured in a wide variety of ways, i.e., for example, such semiconductor device may cam a number of levels of copper layers 34 and dielectric layers 30, 32, with appropriate vias connecting the copper layers.
In this example, for purposes of illustration, the following typical thickness values are given:
During the x-ray inspection, x-rays of a wide range of energies are provided from a source 42 through the tray 22 and into and through the semiconductor device 20, with substantial absorption of x-rays taking place in the copper layers 34, 38 and lead/tin solder balls 36, as compared to the rest of the device, so that proper contrast been the images of the copper layers 34, 38 and solder balls 36 on the one hand, and the rest of the device on the other hand, is provided at image detector 44. In this way, flaws in the copper and/or solder balls can be observed.
During the x-ray inspection process, radiation damage can occur in the silicon body 24. That is, an x-ray beam passing through the silicon body 24 may ionize the silicon, forming electron/hole pairs in the active region 24A, the region approximately 1 xcexcm thick most adjacent the tray 22. These electrons/hole pairs in the active region 24A can cause undesirable changes in device operating characteristics, and can cause changes to stored charge on device internal nodes or within dielectrics, causing improper operation.
For the following discussion, reference is made to pp. 1-17 of ELEMENTS OF X-RAY DIFFRACTION by B. D. Cullity, Addison-Wesley Publishing Co. Inc., published 1956, which material is herein incorporated by reference.
FIG. 2 is a graph showing x-ray absorption coefficient vs. x-ray energy for silicon, copper, tin and lead, with both axes on a logarithmic scale. As will be sen, and as described in that text, for each material, the general trend of the magnitude of absorption coefficient is downward for increasing levels of x-ray energy, varying as the inverse cube of the energy. In addition, as also described in the text, abrupt, distinctive xe2x80x9cedgesxe2x80x9d occur for each element, corresponding to the characteristic K, L, M, etc. lines of the material. As indicated in the graph of FIG. 2, silicon has a high coefficient of absorption in the x-ray energy range of about 3 KeV (and is therefore highly vulnerable to the problem described above). As illustrated in FIG. 2, the absorption coefficient of silicon drops off significantly as x-ray energy increases, so that the vulnerability of the silicon to this problem decreases substantially with increase in x-ray energy.
FIG. 3 is a graphical representation of the structure of FIG. 1, showing x-ray absorption at the 3 KeV energy level (intensity axis on a logarithmic scale, distance axis on a linear scale). As will be seen, after some absorption by the tray 22 and the molding compound 25, the silicon body 24, including the active region 24A thereof, is exposed to x-ray energy of a high intensity and absorbs a substantial amount of x-ray energy at this energy level (the actual absorption of a body is indicated by the change in intensity of the x-ray entering and passing through the body in accordance with the formula.
Ix=I0excexcx
where
xcexc=linear absorption coefficient, dependent on material considered, its density, and the wavelength or energy of the incident x-rays
I0=intensity of incident x-ray beam, and
Ix=intensity of transmitted beam after passing through a thickness x (see the above cited text at page 10).
Even though the active region 24A is only approximately 1 xcexcm thick, as pointed out above, silicon has a high coefficient of absorption at this energy, and there is only the tray 22 and molding compound 25 between the source of x-rays 42 and the active region 24A to absorb x-rays as they travel toward the image detector 44, leading to the problems described above.
With reference to FIG. 4, m the event that the semiconductor device 20 is in a xe2x80x9cflipped overxe2x80x9d state on the tray 22, with substantial absorption of x-ray energy by the material between the active region 24A of the silicon body 24 and the source of x-rays 42, the problem described above is generally avoided (see FIG. 5, even for x-rays passing between the solder balls 36). However, complete structures commonly include semiconductor devices 20 on both sides of a printed circuit board 40, combining the orientation of FIG. 1 and 4 in a single structure, so that the problem described above with regard to the orientation of FIG. 1 continues to exist.
While x-ray inspection system suppliers mention use of a filter in the x-ray process, no systematic approach is indicated for dealing with this problem.
What is needed is an x-ray system wherein the silicon body of a device being x-rayed absorbs minimal x-ray energy, while copper layers and solder balls of the device are highly absorbent of x-ray energy so that proper imaging of the device is provided.
The present apparatus for irradiating a device with x-rays comprising a first material and a second material associated therewith includes a source of x-rays, a filter for receiving x-rays from the source of x-rays and allowing transmission of x-rays therethrough to the device, the filter having an atomic number greater than the atomic number of the second material of the device, and an x-ray imager for receiving x-rays from the device.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.